This invention relates to permanent magnet motors and more particularly to such motors which minimize cogging and simplify rotor position sensing.
Electric motors with permanent magnet rotors suffer from a condition called "cogging." Cogging is a variation in motor torque caused by variations in magnetic flux due to alignment of the rotor and the stator teeth at various positions of the rotor. This effect can reduce the efficiency and reliability of the motor.
Various methods have been tried in the past to overcome the cogging problem. Among these are skewing the magnets on the rotor in either a straight or herringbone pattern. Skewing the magnets in these ways, however, creates additional difficulties. The rotational position of the rotor is typically sensed by a separate sensor such as a Hall effect sensor. To reduce the complexity of the control system for the motor, it is desirable that the sensor be disposed in a particular location with respect to the stator. This position, however, it not necessarily the best when the skewing of the rotor magnets is considered.